Simple Expanded Scale Voltmeter
By Matt Little (matt@re-innovation.co.uk)
August 2007
This document describes the design and practical construction of a very simple
expanded scale voltmeter. This was designed for a renewable energy system in the
developing world, therefore had to be simple, easy to build and cheap. Previously a
standard analogue voltmeter was used on such systems to give an indication of the
system voltage. The scale for such meters was typically 0-50V and the system
(typically a lead-acid battery) voltage could not be read accurately enough for system
diagnostics. The need for a voltmeter that accurately displayed small changes around
the system voltage was highlighted. A design for a 10 to 15V range voltmeter is
shown here but, with a few calculations, any voltage range could be displayed.
Advantages of this are that it is cheap and easy to construct. It is probably not the
most accurate device you could build but it will give better indication of the system
voltage and help with battery diagnostics.
Parts required:
1 x Ammeter (low range is preferred, 10mA in this case)
1 x Resistor
1 x Variable resistor
1 x Zener diode (equal to the lowest voltage to mesure)
1 x Strip board (small piece)
Soldering Iron and solder
Cutter
Screwdriver
Access to a variable power supply
Access to a graphics package and printer would be an advantage
Circuit diagram
The basic idea is to drop some voltage across a zener diode. An ammeter is used to
read the voltage by using a resistor (remember Ohms Law? V=IR). The lowest
voltage that can be read will be equal to the rating of the zener diode (in this case
10V). The scale can then be chosen through the use of the correct resistor values and
some calculation (see below). The variable resistor allows fine tuning of the scale,
especially when the exact resistor values are unobtainable.
The voltage drop across the ammeter and the resistor and part of the variable resistor
is designed to be equal to the highest voltage to be read (Vhigh) minus the rating of the
zener diode (this is also the lowest voltage read, Vlow).
The full scale defection rating of the ammeter can then be used to calculate the
resistance value required. This can then be made from a value resistor that is close but
lower than the resistance required. The variable resistor can then be used, along with
an accurate volt meter to calibrate the device.
Calculations
Definitions:
Zener diode voltage = Vzener
Ammeter maximum current = Imax
Variable resistor = Rvar
Fixed resistor = R
Lowest voltage to measure = Vlow
Highest voltage to measure = Vhigh
1. Choose Vzener = Vlow. This will depend upon the available zener diode ratings.
Zener diodes could be used in series to give the correct value.
2. Voltage across ammeter and resistor network, Vnet = Vhigh – Vzener.
3. Resistance in the path with the ammeter, Rtotal, is comprised of R plus the
proportion of the variable resistor depending upon its position.
Maximum resistance in ammeter path Rtotal max = Rvar + R.
Minimum resistance in ammeter path Rtotal min = R.
4. Apply ohms law (V = IR) to ensure Vhigh can be measured by the chosen
resistor values. The maximum current flowing in this circuit will be equal to
the maximum ammeter full scale deflection value Imax. The equation here is
Vnet = Imax x Rtotal.
5. Using Vnet = Vhigh – Vzener in Vnet = Imax x Rtotal gives the equation:
Vhigh = (Imax x Rtotal) + Vzener
6. The highest Vhigh is when resistance is highest (Vhigh max = Imax (Rvar + R) +
Vzener)
7. The lowest Vhigh is when the resistance is lowest (Vhigh max = Imax (R) + Vzener)
8. This will give the range of values through which the scale can be calibrated.
Let’s put some numbers into that to show what I mean:
1. Vlow = 10V, therefore Vzener must be 10V or the closest available value.
2. Vnet = 16V – 10V = 6V across the ammeter and resistors.
3. Max resistance = 1000 + 100 = 1100 ohm
Min resistance = 100 ohm
4. Imax = 10mA = 0.01A
5. No calculations to do – have a cup of tea?
6. Highest Vhigh = 0.01 x 1100 + 10 = 21V
7. Lowest Vhigh = 0.01 x 100 +10 = 11V
8. Therefore with the values chosen here a voltmeter with a low reading of 10V
and a high reading of 11V to 21V can be produced.
Obviously just adjust the values above to whatever you have to hand or whatever
range you require.
For a 12V nominal lead-acid battery the battery voltage range of interest is from about
10V to 16V DC. This can be multiplied up for other system voltages (eg 48V system
should read from 40V to 64V)
This voltmeter will draw slightly over 10mA to read the voltage. Relatively, this is a
very small load (about the same as an LED), but it will be a constant load. A switch
could be added but I would not recommend it as it good to ensure the battery voltage
is always visible. For a renewable energy system with a large battery bank this load
will be negligible.
Practical construction
Build the circuit on a small piece of strip board:
Open up the ammeter and carefully de-solder and remove the coil:
Drill a hole large enough for the variable resistor to turn. Glue the strip board in place
so that the variable resistor is accessible, with a screwdriver, through the hole:
Create a new scale for the device. This can either be done by hand (paint onto the
original scale or draw a new one on a piece of paper) or using some graphical design
package and a printer. In this case Corel Draw was used.
No colour printer available – hence the black and white dial
Final device testing and calibration
The device then needs calibrating. Use an accurate voltmeter or Multimeter and a
variable power supply to do this. Set the variable resistor to its highest value. Apply
the lowest voltage (measure with voltmeter) and ensure that the dial reads 0mA or
Vlow. Then apply the highest voltage Vhigh (measure with voltmeter). Adjust the
variable resistor until the meter needle reads the correct value. Test the unit to ensure
that, within the required range, the meter reads correctly. There may be problems with
the linearity.
Adjust variable resistor until the dial value is the same as the calibrated Multimeter.
Check the scale by running through the range of voltages.